As undercoating of metal materials or the like, there is generally employed cationic electrodeposition having high corrosion resistance. Furthermore, as a special use, cationic electrodeposition may be utilized in order to form an electric insulating film on the metal material. As a cationic electrocoating composition used in such the cationic electrodeposition, there is known a water-borne resin composition, which contains an epoxy resin as a base resin and has a hydrolysable functional group and a polymerizable unsaturated carbon bond (cf. Japanese Kokai Publication 2000-038525). In order to obtain a film having adequate heat resistance using such a resin, it is required to form a cured film having a high crosslinking density. Therefore, there were cases where the flexibility of the obtained film became insufficient and the processability of the film was reduced.
On the other hand, there is generally disclosed an electric insulating coating composition using a polyamide-imide resin, a polyamide resin and a polyimide resin as a base resin (cf. Japanese Kokai Publication Hei-05-295324 and Japanese Kokai Publication 2001-351441). It is known that these base resins can give a film having good heat resistance and excellent flexibility even if a cured film having a high crosslinking density is not necessarily formed because of high heat resistance of the resin itself. However, these electric insulating coating compositions contain organic solvent in large amounts and have a problem from the viewpoint of environmental protection.
There are disclosed electrocoating compositions, which are water-borne coatings and contain a polyimide resin, having such a function, as a base resin (cf. Japanese Kokai Publication Hei-09-124978 and Japanese Kokai Publication 2002-38078). However, since such the electrocoating compositions had insufficient stability, they were not substances capable of with standing long-term uses. And the obtained film did not have a sufficient film thickness and a satisfactory electrical insulating property.